Add projects paths to the compare result

As pointed by @kwridan in https://github.com/bloomberg/xcdiff/pull/56#pullrequestreview-418000411, it might be useful to have paths to `-p1`, `-p2` projects in the result type. The paths could be included in supported formats (not only HTML).

I wonder if there is anything else we could add to the header section, i.e.
- date of the generation
- total number of detected differences

Start by locating the compare result type and the implementations for each supported output format. Determine which project paths and header metadata are in scope, then update the relevant formats and verify that the generated results expose the agreed information consistently.

Written by the indexing model from the issue text.